16:Kinetics II
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What does the rate of reaction measure?
How quickly a chemical change takes place.
How is reaction rate usually expressed?
As the change in concentration of a reactant (or product) per unit time, e.g., mol dm⁻³ s⁻¹.
What is the general form of the rate equation?
rate = k[A]ᵐ[B]ⁿ
In the rate equation (rate = k[A]ᵐ[B]ⁿ), what does k represent?
The rate constant, which varies with temperature.
In the rate equation (rate = k[A]ᵐ[B]ⁿ), what does m and n represent?
The orders of reaction with respect to each reactant.
What does the order of reaction tell us?
It tells us how the rate responds to changes in the concentration of a reactant.
What happens in a 0-order reaction if the concentration of the reactant changes?
Changing the concentration of the reactant does not affect the rate.
What happens in a 1st-order reaction if the concentration of the reactant changes?
Changing the concentration of the reactant changes the rate by the same factor (e.g., rate doubles if concentration doubles).
What happens in a 2nd-order reaction if the concentration of the reactant changes?
Changing the concentration of the reactant changes the rate by the factor squared (e.g., rate quadruples if concentration doubles)
How can the orders of reactions be determined?
They can only be determined experimentally
How is the overall order of a reaction calculated?
It is the sum of all the individual orders with respect to each reactant.
Why are different experimental techniques used to measure reaction rates?
Different techniques are used to determine reaction orders experimentally, depending on the type of reaction.
What is the titration (“quenching”) method for measuring reaction rates?
Samples are taken at regular intervals, quenched (e.g., cooled), and titrated to find concentrations. Suitable for acid–base or redox reactions.
When is the mass loss method useful for measuring reaction rates?
When a gas is released during the reaction; the loss in mass over time is recorded.
How does colorimetry measure reaction rates?
By measuring the colour change of a solution over time.
How does the gas volume method measure reaction rates?
Using gas syringes or inverted burettes to measure the volume of gas formed during the reaction
What can be measured to determine reaction order using the initial rate method?
How the initial reaction rate changes when reactant concentrations are varied.
How do you carry out the initial rate method?
1) Carry out the reaction multiple times, varying one reactant concentration at a time. 2) Measure the initial rate for each experiment. 3) Compare how the rate changes to determine reaction order.
In a rate against concentration graph, how can you determine order by using the shape?
• Zero order: Vertical line • First order: Diagonal line through the origin • Second order: Curves going upwards
How can you determine zero order from a concentration-time graph?
Shape is linear (constant rate) - basically diagonal but downwards
How can you determine first order from a concentration-time graph?
There is exponential decay
How can you determine second order from a concentration-time graph?
There is a steep exponential decay
Do most chemical reactions happen in a single step or multiple steps?
Most reactions occur in a series of steps, not a single step.
Do all steps in a reaction mechanism occur at the same rate?
No, different steps can occur at different rates.